1. Field of the Invention
The present invention relates to integrated transformers, and more particularly, to an integrated transformer in which a power conversion transformer and an inductor for stabilizing output power by the transformer are integrated into one transformer structure, and a power supply using the same.
2. Description of the Related Art
In general, power supplies that convert commercial AC power into DC power and provide the DC power are widely used in home appliances, such as information devices including personal computers, air conditioners, and audio and visual devices.
With the development of home appliances, power supplies that output DC power of hundreds of watts (W) or more are required. As described above, while the power supply outputs high DC power, the power supplies also need to be small, lightweight, and thin.
In order to satisfy the needs, a transformer that is a magnetic element used for power conversion and an output inductor that stabilizes the converted power in the power supply need to be integrated.
FIG. 1 is a view illustrating an integrated transformer structure into which a transformer and an output inductor are integrated according to the related art.
Referring to FIG. 1, an integrated transformer 1 according to the related art includes a magnetic core and windings.
The magnetic core includes first and second supports 1a and 1b, first and second outer legs 1c and 1d, and a center leg 1e. Each of the first and second supports 1a and 1b facing each other includes one end and the other end and has a predetermined length. The first and second outer legs 1c and 1d and the center leg 1e extend from the first and second supports 1a and 1b, and magnetically couple the first and second supports 1a and 1b to each other.
The windings include primary windings P1 and P2, secondary windings S1 and S2, and an inductor coil Lo, and the secondary windings S1 and S2 are electromagnetically coupled to the primary windings P1 and P2, respectively. The inductor coil Lo corresponds to the above-described output inductor. The primary windings P1 and P2 and the secondary windings S1 and S2 correspond to the power conversion transformer.
The primary winding P1 and the secondary winding S are wound on the first outer leg 1c. The primary winding P2 and the secondary winding S2 are wound on the second outer leg 1d. An inductor winding of the inductor coil Lo is wound on the center leg 1e. 
The integrated transformer according to the related art has disadvantages as follows.
First, a commercial EE core or a commercial EI core is used as the magnetic core. The primary windings P1 and P2 and the secondary windings S and S2 that are used to perform power conversion are wound on the outer legs 1c and 1d having a smaller cross-sectional area than the center leg 1e. DC magnetic flux generated by the inductor winding Lo of the center leg 1e and AC magnetic flux generated by the primary windings P1 and P2 and the secondary windings S1 and S2 are put together to significantly increase magnetic flux density of the outer legs 1c and 1d. Since DC components are included in the magnetic flux of the primary windings P1 and P2 and the secondary windings S1 and S2, the transformer can be easily magnetically saturated by AC asymmetrical operation or a small asymmetrical circuit element of the power supply.
Second, since the inductor winding Lo is only wound on the center leg 1e, the magnetic flux generated by the inductor flows through the center leg 1e, and the center leg 1e has a larger cross-sectional area than that of the outer legs 1c and 1d, which causes very low core loss. Therefore, the most amount of core loss occurs in the outer legs 1c and 1d on which the primary windings P1 and P2 and the secondary windings S1 and S2 are wound. As a result, thermal unbalance occurs between the outer legs 1c and 1d and the center leg 1e. 
Third, since the primary windings P1 and P2 and the secondary windings S1 and S2 are wound around the outer legs 1c and 1d, when the number of turns of the windings is high, the magnetic flux may leak to the outside since there is no core to wound on, which may cause electromagnetic interference. Further, since the windings of the transformer are divided into two, a coupling coefficient between the primary windings P1 and P2 and the secondary windings S1 and S2 is low.